1. Field
This invention relates to a radio antenna suitable for fitting to a motor vehicle such as a car, and in particular, to an antenna which is able to receive digital audio broadcast (DAB) transmissions for use with a DAB receiver positioned within the vehicle.
2. Related Art
Digital audio broadcasting (DAB) is now well established in the United Kingdom and various other European countries and is also now becoming better established in further countries worldwide. It is expected that over a number of years, there will be a switch off of FM transmissions in countries where DAB becomes established.
Many motor vehicles are equipped with FM/AM radio receivers and have antennas only capable of receiving signals for these frequencies, for example, FM frequency signals are in the band 87.5 MHz to 108 MHz. DAB transmissions come in two frequency bands, band 3 is 174 MHz to 240 MHz, and band L is 1452 MHz to 1490 MHz. Due to the different frequency bands, if the radio receiver in a vehicle is to be upgraded to a DAB receiver from an FM/AM receiver then a different antenna will be required on that vehicle to receive signals. Several different Antennas for after-market fit to vehicles have already been proposed and all of these have drawbacks.
Windscreen film antennas are antennas printed on plastic film which are stuck to the inside of a vehicle's windscreen, they can be active or passive antennas. Although they are aesthetically fairly discreet, they suffer from performance issues which arise mainly because they are mounted on the inside of a vehicle windshield. Due to the mounting position they suffer shadowing from the roof and the rest of the structure of the vehicle, which will result in a reduction in nominal performance and a further reduction still depending on the vehicle's orientation to the transmitter. Furthermore, some windscreens have coatings to reflect UV or glare and these coatings can themselves effect reception of an antenna fixed to the inside of the windscreen.
Magmount antennas are usually mounted on the roof of a vehicle. They are a monopole structure and the antenna base is a magnet and is therefore magnetically attracted to the roof. The position of the antenna above the roofline gives this antenna good performance. However, the antenna requires a coaxial cable to connect to a radio receiver inside the vehicle and this will require some routing through a window seal or by making a hole in the bodywork of the vehicle.
External glass-mounted antennas are mounted outside the vehicle and therefore above the roofline, this gives them good omnidirectional performance. However, the antenna being monopole in structure requires a ground plane if it is to provide good performance, particularly with Band 3 reception which covers a wide frequency bandwidth. A further performance reduction arises because of the loss of signal incurred when coupling the signal through the glass. This effects the sensitivity of the receiver as there is no amplification of the signal prior to this loss and therefore the signal to noise ratio (SNR) of the received signal will be compromised.